1. Field of the Invention
This invention relates to a method and apparatus for supporting particles onto a granular support, particularly, a grain size of a granular support is equal to or less than 1 μm and a particle size of a supported particle is equal to or less than 10 nm.
2. Description of Related Art
Noble metals such as Pt are used not only as an accessory but also as a chemical catalyst. For example, an exhaust emission control system and a solid high polymer fuel cell use Pt as a catalyst. In order to apply a fuel cell to a mobile device and so on as a power source, a fuel cell which uses methanol solution as a fuel is studied actively because it can operate in low temperature and is small and lightweight. However, for a fuel cell to be widely used, further improvement of a fuel cell in performance is desired. A fuel cell converts chemical energy which is generated by an electrocatalytic reaction to electric power, it needs high-activity catalyst to achieve a high efficiency.
PtRu is widely used as a catalyst of an anode of a fuel cell. Though theoretical output voltage of an electrocatalytic reaction is 1.21 V, a voltage loss occurs at a PtRu catalyst. A voltage loss of a PtRu catalyst is about 0.3 V. A high-activity (methanol oxidization activity) which is higher than that of PtRu as an anode catalyst is desired in order to decrease the voltage loss at an anode catalyst.
In order to improve a methanol oxidization activity, addition of elements except for Pt and Ru to a PtRu catalyst is studied. In a method to produce a catalyst using a solution such as dipping method, metal elements (elements used as a catalyst) to be supported is oxidized and deposited onto a surface of a carbon granular support. The deposited metal elements are reduced and a metal catalyst is obtained. Therefore, a heat treatment has to be performed in a reduction gas atmosphere, and a temperature of the heat treatment is different based on an element.
In case that Pt and Ru are used, Pt and Ru are reduced at almost the same temperature, and Pt and Ru are easily alloyed. However, if other elements that improve a catalyst activity are added, a reduction temperature may be extremely high, and a reaction between a carbon which is a support base and elements which are included as a catalyst may occur.
A production of a catalyst using a sputtering method or an evaporation method is studied. According to these methods, objective elements are directly deposited onto a carbon support base. Therefore, deposited elements do not have to be reduced, and PtRu alloy is formed even in a room temperature.
In a conventional sputtering method or an evaporation method, carbon which is formed like a sheet (hereinafter it is called a carbon paper) is generally used. In this case, catalyst particles are deposited on a surface of a carbon paper. In such a case, catalyst particles are deposited only on a surface of a carbon paper. Therefore, when a few nm of catalyst particles are deposited, an amount of catalyst which is supported by a carbon paper is not enough to obtain necessary electric power to be generated. Further, catalyst particles may not be formed and a thin film of a catalyst may be formed due to a situation of a deposition. If a thin film of a catalyst is formed, a surface area of a catalyst becomes narrow and electric-generation capacity is decreased.
On the other hand, it is known that catalyst metal materials can be deposited on a granular support (Please refer to Japanese Unexamined Patent Publication Number 2005-264297).
In the above described document, catalyst elements are deposited on carbon granular supports while the carbon granular supports are stirred. In case that the catalyst elements are deposited on carbon granular supports while the carbon granular supports are stirred, it is difficult to detect material other than carbon even if the products are observed by an electronic microscope. The difficulty of detection is mainly caused by two reasons. One reason depends on a forming process for evaporated atoms to form metal particles, and another reason depends on a surface condition of carbon granular supports on which catalyst particles are deposited. In case that metal atoms (catalyst atoms) are physically deposited using an evaporation method in vacuum condition, objective elements are evaporated and atoms of objective elements are deposited onto granular supports (For example, carbon) by using a thermal and kinetic energy. Evaporated metal atoms migrate at a portion where energetically stable level, and deposited metal atoms form seeds of alloy. Particles of a catalyst alloy are grown using the seeds above described, and grown-up particles are united with each other and form a polycrystalline film.
In case that the diameter of a carbon granular support is equal to or less than 1 μm, the surface of the carbon granular support has a lot of defects. Therefore, in a carbon granular support which has a diameter less than 1 μm, an area of the surface on which the evaporated atoms can be migrated becomes narrow and seeds to become catalyst particle are hardly formed. If catalyst atoms are deposited onto carbon granular supports while the carbon granular supports are stirred, carbon granular supports are removed before the seeds of catalyst particles are formed. Therefore, the seeds and particles of the catalyst are not formed because only catalyst atoms are attached onto carbon granular supports. In order to achieve a function as a catalyst, it is preferable that a diameter of a catalyst particle to have a diameter between 2 nm to 10 nm. Therefore, an atomic catalyst itself is not able to be used as a catalyst.
Further, when a thin film of a catalyst alloy is formed on carbon supports using a sputtering method, an alloy target corresponding to an objective alloy is generally used. However, a target made of noble metals has disadvantages in material costs and processing costs. Further, a target that is used in a sputtering method has a region called an erosion region. The erosion region is a region which has a higher etching rate than other region of a target. A hole that penetrates the target may be formed if the erosion region is sputtered. The target having a hole is not able to be used as a target. Therefore, a target corresponding to the composition of the catalyst alloy is only used 10% to 20% of a whole target amount. The target which can not use as a sputtering target is recycled by melting the remaining material. However, in order to recycle a remaining target, additional costs are needed. For example, processing costs, analyzing costs of a remaining target, and so on. Further, if an element which hardly melt is included in a catalyst alloy, sputtering rate of the element may vary while a sputtering is performed, and a composition of the deposited alloy may be varied.
According to the present invention, a granular support which supports alloy particles can be obtained. Particularly, a diameter of the alloy particles is equal to or wider than 2 μm and equal to or narrower than 10 μm, and a cost for producing granular supports is decreased and a fluctuation of a composition of a catalyst alloy is decreased.